(1) Field of the Invention
The present invention relates to a method for working a nickel-base alloy and more particularly to a thermomechanical treatment which is able to introduce superplasticity to the alloy.
(2) Description of the Related Art
It is known that .gamma.-precipitation hardening-type nickel-base alloys cannot be forged on account of their extremely high strength, their recrystallization temperature close to their melting point, and their extremely low ductility, and consequently they are formed by precision casting, whereas they exhibit superplasticity and enhanced ductility when their crystal grains are reduced in size. A nickel-base alloy of fine crystal grains is produced by powder metallurgy because it is impossible to reduce the size of crystal grains by ordinary melt casting. Recently, a nickel-base alloy having fine crystal grains has been produced by the roll method which includes the step of pouring a molten metal onto the surface of a roll running at a high speed.
The superplasticity of a nickel-base alloy manifests itself when it is composed of fine crystal grains. The finer the crystal grains, the better the characteristic properties of the alloy. The grain refinement is not achieved by the conventional powder metallurgy, and a structure of fine grains can be obtained only by large-scale preforming such as HIP or hot extrusion. This leads to a very high production cost. On the other hand, the roll method that brings about rapid solidification can be applied only to the production of thin tape (about 100.mu.m), and it cannot be applied to the production of thick sheet for sheet working and isothermal forging. Therefore, the application of superplasticity has been extremely limited.
Conventional nickel-base alloys (such as IN 100 which exhibit superplasticity have a hardness of about Hv 450 if they undergo precipitation hardening without work hardening after solution treatment. This hardness is not sufficient for them to be used as ornamental hard alloys. To make the alloy convert into an ornamental hard alloy having a hardness of about Hv 600 by precipitation hardening, it should undergo cold working such as sizing after superplasticity working, because superplasticity is abnormal ductility accompanied by work softening and superplasticity does not increase hardness. For this reason, superplastic working is only possible to near netshape, and it has been impossible to apply the transcription ability, which is one of the characteristic properties of superplasticity, to the nickel-base alloy of precipitation hardening type.
In addition, a disadvantage of nickel-base alloys containing nickel 58-72%, chromium 25-35%, and aluminum 3.0-7.0% is that they are capable of deformation in their solution state but they have a high deformation stress. This makes it necessary to install a large equipment for forming of complicated objects such as a watch case, except forming of simple plates and rods. An additional disadvantage is that the solid solution temperature of the precipitation phase is about 1000.degree. C. If the hot working is performed at a temperature lower than that, cracking caused by the presence of precipitates is liable to occur at the precipitate. If the hot working is performed at a temperature higher than that, grains grow so rapidly that hot working is difficult to carry out.